This invention relates to electrophotographic imaging members and more specifically, to imaging members having characteristics that enable high quality and high contrast imaging.
Electrophotographic photoreceptors typically include a photoconductive layer formed on a conductive substrate. The photoconductive layer is a good insulator in the dark so that electric charges can be retained on its surface. But upon exposure to light the charge is dissipated.
A latent image is formed on the photoreceptor by first uniformly depositing electric charges over the surface of the photoconductive layer by a conventional means. The photoconductive layer acts as a charge storage capacitor with charge on its free surface and an equal charge of opposite polarity (the counter charge) on the conductive substrate. A light image is then projected onto the photoconductive layer. On those portions of the photoconductive layer that are exposed to light, the electric charge is conducted through the layer reducing the surface charge. The portions of the photoconductive surface not exposed to light retain their surface charge. The quantity of electric charge at any particular area of the photoconductive surface is inversely related to the illumination incident thereon, thus forming a latent electrostatic image.
The photodischarge of the photoconductive layer requires the layer to photogenerate conductive charge and to transport this charge through the layer thereby neutralizing the charge on the surface. Two types of photoreceptor structures have been employed: Multilayer structures wherein separate layers perform the functions of charge generation and charge transport, respectively, and single layer photoconductors which perform both functions. These layers are laminated onto a conducting substrate and may include an optional charge blocking and an adhesive layer between the conducting and the photoconducting layers. Additionally, they may contain protective overcoatings and the substrate may consist of a non-conducting mechanical support with a conductive layer. Other layers to provide special functions such as incoherent reflection of laser light, dot patterns for pictorial imaging or subbing layers to provide chemical sealing and/or a smooth coating surface may be employed.
One common type of photoreceptor is a multilayered device that comprises a conductive layer, a blocking layer, an adhesive layer, a charge generating layer, and a charge transport layer. The charge transport layer can contain an active aromatic diamine molecule, which enables charge transport, dissolved or molecularly dispersed in a film forming binder. This type of charge transport layer is described, for example in U.S. Pat. No. 4,265,990. Other charge transport molecules disclosed in the prior art include a variety of electron donor, aromatic amines, oxadiazoles, oxazoles, hydrazones and stilbenes for hole transport and electron acceptor molecules for electron transport. Other charge transport layers have been developed that employ a charge transporting polymer wherein the charge transporting moiety is incorporated in the polymer as a pendant or in the chain or may form the backbone of the polymer. This type of charge transport polymer includes materials such as poly N-vinylcarbazole), polysilylenes, and others including those described in U.S. Pat. Nos. 4,618,551, 4,806,443, 4,806,444, 4,818,650, 4,935,487, and 4,956,440.
Charge generator layers employed include amorphous films of selenium and alloys of selenium and arsenic, tellurium, germanium and the like, hydrogenetated amorphous silicon and compounds of silicon and germanium, carbon, oxygen, nitrogen and the like, fabricated by vacuum evaporation or deposition, inorganic pigments of crystalline selenium and its alloys, III-V and II-VI compounds and organic pigments such as quinacridones, polycyclic pigments such as dibromo anthanthrone pigments, perylene and perinone diamines, polynuclear aromatic quinones, azo pigments including bis-, tris- and tetrakis-azos, and the like dispersed in a film forming polymeric binder and fabricated by solvent coating.
Phthalocyanines have been employed as photogenerating materials for use in laser printers with infrared exposures. Infra red sensitivity is required for low cost semiconductor laser diodes used as the light exposure source. The absorption spectrum and photosensitivity depend on the central metal atom. Many metal phthalocyanines have been reported and include, oxyvanadium phthalocyanine, chloroaluminum phthalocyanine, copper phthalocyanine, oxytitanium phthalocyanine, chlorogallium phthalocyanine, magnesium phthalocyanine and metal-free phthalocyanine. The phthalocyanines exist in many crystal forms which have a strong influence on photogeneration.
Single layer photoreceptors commonly employed include photoconducting layers laminated onto a conducting substrate and may also include an optional charge blocking and/or an adhesive layer between the conducting and the photoconducting layers. The photoconducting layer materials include amorphous selenium and alloys of selenium and arsenic, tellurium, germanium and the like, hydrogenetated amorphous silicon and compounds of silicon and germanium, carbon, oxygen nitrogen and the like fabricated by vacuum evaporation or deposition, inorganic pigments of crystalline selenium and its alloys, II-VI crystals such as ZnO, CdS, III-V pigments and the like and organic pigments such as quinacridones, polycyclic pigments such as dibromo anthanthrone pigments, perylene and perinone diamines, polynuclear aromatic quinones, azo pigments including bis-, tris- and tetrakis-azos, metal phthalocyanines and the like dispersed in a film forming polymeric binder fabricated by solvent coating. Other organic photoconductor materials are electron donor and acceptor charge transfer systems such as polyvinyl-carbazole (PVK), 2,4,7-trinitro-9-fluorenone (TNF) and the like. The pure pigment photoconducting layers, such as amorphous selenium and silicon, both photogenerate and transport a charge. But when the pigment in is a binder layer, charge transport may take place entirely within the pigment while the binder is substantially insulating, as for example in the ZnO photoreceptor. Alternatively, charge transport may occur in a binder which is either (a) an insulating polymer doped with (i) an electron donor or (ii) acceptor molecules or (b) a charge transporting polymer as described above.
Charge generation controls the discharge (both photo and dark) of all the dual layer and nearly all the single layer photoreceptors. Restated, the amount of charge neutralized, as measured by the voltage across the photoconducting layers, is proportional to the light exposure (e.g., ergs/cm.sup.2). The photodischarge curve is linear with a negative slope from the maximum (dark or zero exposure) voltage to the minimum voltage. The minimum voltage is referred to as the residual voltage. Light exposure beyond that required to reach the residual voltage does not produce any further discharge. In such photogeneration limited discharge, the ideal discharge is a linear discharge down to zero (residual) voltage with the slope being a measure of the photosensitivity. However, because the photogeneration rate in practical materials is electric field dependent, and decreasing with field, the discharge slope decreases and the discharge curve at low voltages increasingly departs from the linear discharge, requiring increasingly more light exposure to the same voltage discharge, as shown in FIG. 1. Because dark discharge, which is undesirable, also is generation limited, albeit thermal generation limited, dark discharge has the same electric field dependence, being high at high voltages (electric fields) and low at low voltages (electric fields).
Generation limited discharge is undesirable because it contributes to undesirable image quality variation through variations in electricals, that is, the voltages on the photoreceptor. Highest image quality in a xerographic system requires the voltages corresponding to the same image density or white background be constant, both spatially across the entire copy or print and temporally (or cyclically) from print to print. The generation limited discharge contributes to electrical variation in two ways. First, small variations at low light exposure result in large variations in the high (dark) voltage. Secondly, small variations in thermal generation also cause variation in the high (dark) voltage. The previous solutions have been to improve the materials and coating technologies to reduce the electrical variation of photoreceptors and improve the optics and electrical controls in the xerographic imaging machines.
Digital imaging provides an improvement in image quality. Digital systems have been used where gray or tone scales are produced by area coverage at constant local image density. Thus it is desirable to have a discharge curve (both photo and dark discharge if possible) that appears as a switch, with negligible voltage discharge until a critical exposure is reached, followed by complete discharge to residual voltage. This type of discharge is called S shaped hereinafter, as shown in FIG. 2. Such a binary discharge curve permits variation in both the off (or dark) and on (or fully exposed) light exposure with negligible voltage variation. Additionally, dark charge generation does not cause a dark voltage variation contributing to stable electricals.
One approach is to fabricate a single-layer, heterogeneous, particle-contact device in which photoconductor pigments are dispersed in insulating binders. The concentration of the charge generating and transporting pigment particles is high enough to maintain particle contact and thus a conducting path through the layer.
The key to an S shaped photodischarge curve is a heterogeneous structure which provides a connected but convoluted path for charge transport or conduction. At high electric fields, after the sample is charged, any charge generated at the surface is directed in a straight line through the layer, encounters a barrier in the insulating region and hence causes negligible voltage discharge. After nearly all the surface charge is injected, the local electric field normal to the surface is negligible and the remaining charge is able to move in other directions and follow the connected path to a depth below where the initial charge was stopped. At this deeper level the charge again sees the full electric field and encounters the insulating barrier. But because the motion of the previous charge reduced the electric field in the first level, more charge follows the convoluted path down to the next level. Thus by such a cascade total discharge occurs after a light exposure corresponding to the generation of enough charge required for total discharge, resulting in a step-like or S shaped discharge curve. By a similar argument, the dark discharge also has an S shaped time dependence, enabling very stable dark potentials.
The earliest such devise with an S shaped photodischarge curve is the single layer ZnO electrophotographic layer.
Another single layer device with S shaped photodischarge is described by J. W. Weigl et al. in "Current Problems in Electrophotography", pages 286-300, edited by W. F. Berg and K. Hauffe and published by Walter de Gruyter, Berlin in 1972. The layers consist of microcrystalline dispersions of X-metal free phthalocyanine in suitable binders. The X-metal free phthalocyanine, which are observed as needle like crystals, provides both the photogeneration and the hole transport in this device.
Another single layer particle contact device is discussed in articles "An aggregate Organic Photoconductor Part 1 and 2" by Dullmage et al. and Borsenberger et al. and is published in the Journal of Applied Physics, Vol 4, pages 5555-5564, 1978. The device described is a two phase aggregate photoconductor containing a co-crystalline phase of a thiopyrylium dye and a polycarbonate polymer in an amorphous phase of a triphenylmethane derivative in polycarbonate. An S shaped discharge shape is observed when the device is charged negatively and discharged by highly absorbed light. When charged positively, the normal generation limited discharge is observed. The photogeneration is attributed to the thiopyrylium and the discharge proceeds by hole transport through the amorphous phase of the triphenylmethane hole transport molecules in polycarbonate. When charged negatively, the discharge proceeds by electron transport through the co-crystalline phase, which form a dendritic network.
In the prior art, the S shaped discharge is observed in single layer devices which suffer from inflexibility in design. The same material, a pigment, is employed to photogenerate and transport the charge.
It is, therefore, an object of the present invention to provide an improved electrophotographic imaging member which overcomes the above noted disadvantages.
It is another object of the present invention to provide an electrophotographic imaging member whose discharge shape is controlled by the heterogeneous properties of the transport layer.
It is still another object of the present invention to provide an electrophotographic imaging member which exhibits an S shaped, high contrast discharge in a multi layer photoconductor.
It is still another object of this invention to provide a multilayer photoreceptor in which the transport layer has a structure in which charge transporting regions are surrounded by non-transporting regions and the charge transporting regions are in contact with each other, forming a convoluted path across the layer.